I picked up a used 600 f/4G for a decent price this last November.
It is incredible and has really been a huge step up from the Nikon 200-500 (with the Kirk long lens support) that I was using for wildlife. The 200-500 will probably still get used for kids sports.
I have the 1.4III, 1.7II and 2.0III teleconverters. The quality loss with the 1.4 isn't too bad, the 1.7 and 2.0 are each progressively worse by about the same amount.
I am using them on a D810 and a D7200.
As the magnification is increased the stability of the mount, etc becomes more and more important to remove the motion blur and the focal plane can be very thin even at pretty good distances so it can be really hard to nail focus on something like a duck on a choppy body of water a few hundred yards away.
I am using the Induro GIT404KL tripod and Wimberley WH-200 II gimbal head. I also use the Really Right Stuff long lens support (goes "backwards" on the Nikon since the foot on the G is towards the front of the lens).
I decided that I was going to get a large prime after a trip to Japan where the guide loaned me his 300 f/2.8G to take pictures of a rare owl at night. The lens so far outperformed everything I had known before (as a former Canon user and current Nikon user) that I knew I had to get me something for myself. Since with the birds always seem to need more magnification I got the 600g since it was the most affordable. I would have gotten the 800, but the price is just way too out there.
The 600 has totally changed my wildlife photography. When people say that the equipment doesn't matter they are correct to a point, but there is a reason this equipment exists. It makes a difference at the high end.
300 f/2.8G
600 f/4G